Surface-acoustic-wave convolver

ABSTRACT

Disclosed is a surface-acoustic-wave convolver having a toroidal coil connected to each interdigital electrode as a matching and balanced-to-unbalanced converting circuit. The use of the toroidal coil wound on an associated toroidal core permits the reduction of required parts in number thanks to its dual function, and accordingly reduction of manufacturing cost and size of the device. Still advantageously the use of such a toroidal winding permits the suppression of spurious or noise signals in an SAW device.

This is a continuation Ser. No. 07/255,103 filed on Oct. 7, 1988, nowU.S. Pat. No. 4,950,935 issued Aug. 21, 1990.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface-acoustic-wave convolver, andmore particularly to a matching circuit to be associated with asurface-acoustic-wave device (hereinafter abbreviated as SAW device.)

2. Description of the Prior Art

A matching circuit having an inductance L, a capacitance C and aresistance R is used for the purpose of matching on SAW device to anassociated exterior circuit. For example, Japanese Patent ApplicationPublic Disclosure No. 56-156015 shows an SAW device having an inductanceconnected thereacross. This arrangement can cancel the imaginary part ofthe admittance of the SAW device. In order to effect the necessarymatching of the real part of the admittance of the SAW device, forinstance, the conductance of 50 ohms, however, it is necessary that theradiation conductance G of the electrode is designed to be equal to 20mS, and accordingly the electrode opening must be enlarged. As a resultthe piezoelectric substrate is enlarged in area.

Japanese Patent Application Public Disclosure No. 61-230512 shows that aseries-and-parallel arrangement of capacitive and inductive elementsconstitutes a matching circuit, which can cancel the imaginary part ofthe admittance of the SAW device and at the same time, can effect thematching of the real part of the admittance of the SAW device. Theprinting of the series-and-parallel arrangement of capacitive andinductive elements on a piezoelectric substrate will disadvantageouslycause enlargement of the substrate.

There are two different feeding systems to the opposite interdigitaltransducers (hereinafter abbreviated as IDT) of SAW device, balancefeeding system and unbalance feeding system. These different systemshave merits and demerits. Specifically, the balance feeding system inwhich the opposite IDTs are electromagnetically coupled in balance, hasa cancelling effect, thus advantageously preventing spurious signals.However, it is necessary to provide a balanced-to-unbalanced transformerbetween each IDT of the SAW device and an exterior signal channel, whichis generally of unbalance type. In contrast to this, the unbalancefeeding system requires no balanced-to-unbalanced transformer, but theremust be provided extra means to suppress spurious signals.

The admittance of the IDT formed on the piezoelectric substrate is givenat its center frequency as follows:

    Y=G+j ωC.sub.r

C_(r) depends on how the IDT is formed on the piezoelectric substrate.Specifically, assume that the IDT is formed on a thin piezoelectricsubstrate having a conductive lining on its bottom surface, and then ina balance feeding system if used, the value of C_(r) is reduced twotimes as much as that in forming the IDT on a thick piezoelectricsubstrate having no conductive lining at its bottom. Such reduction isattributable to the grounding function of the lining conductor forhigh-frequency signals. Also, the former arrangement causes an increaseof the electro-mechanical coupling coefficient K^(Z). Stilladvantageously, the balance feeding which the former arrangement adopts,has an effect to suppress spurious signals.

Usually in an SAW convolver having a piezoelectric substrate such asZ_(n), O substrate, a balance feeding system is used for the abovedescribed reason, and also a matching circuit and abalanced-to-unbalanced transformer are used. The use of the matchingcircuit and the balanced-to-unbalanced transformer, however, preventsdisadvantageously the reduction of the size and cost of the device.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an SAW device which isfree of the defects described above, permitting the expansion offrequency band and reduction of loss.

To attain this object an SAW device according to the present inventioncomprises a semiconductor base, a piezoelectric substrate laid on thesemiconductor base, at least one transducer laid on the piezoelectricsubstrate, and an associated matching circuit essentially comprising atransmission line which has an inductance component to cancel thesusceptance ωC_(r) of the transducer, and has a capability ofbalance-to-unbalance conversion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an SAW convolver according to thepresent invention.

FIG. 2 is a Smith Chart showing the admittance of an IDT.

FIG. 3 shows an equivalent circuit of an IDT.

FIG. 4 shows an distributed constant circuit.

FIG. 5 (a) and 5 (b) show IDTs each connected to a different typetransmission line.

FIG. 6 shows a transmission line transformer.

FIG. 7 shows a transmission line-like transformer.

FIG. 8 is a circuit diagram of an IDT connected to a transmissionline-like transformer.

FIG. 9 shows schematically a transmission line-like transformer.

FIG. 10 shows schematically another transmission line-like transformer.

First, the principles of the present invention is described.

Generally, the admittance Y of the IDT is given at its center frequencyas Y=G+jωC_(r), where G stands for radiation conductance and C_(r)stands for the capacitance of the IDT electrode. Its frequencycharacteristic is indicated at "a" in the Smith Chart of FIG. 2.

Assume that an inductance L is series-connected to cancel thecapacitance C_(r) of the IDT as shown in FIG. 3, and then, the impedanceof the IDT will be resistance R. The frequency characteristic isindicated at "b" in FIG. 2.

Referring to FIG. 4, there is shown a transmission line (characteristicimpedance: Z_(o)) having a load Z_(L) at its receiving end. Theimpedance Z_(in) as seen from the sending end of the transmission lineis given by: ##EQU1##

If the length of the transmission line is selected to be equal to thefourth of the wave length at the frequency of the acoustic wave (n/4),the above equation will be reduced to Z_(in) =Z_(o) ² /Z_(L). For realnumbers of impedance Z_(o) and load impedance Z_(L) and for Z_(in) =50ohms, the characteristic impedance Z_(o) of the Transmission line willbe given by: ##EQU2## It also follows from the above equation, as iswell known to those knowledgeable in the art, that if the load impedanceis in the form of a resistor shunted by a capacitor, the input impedancemay be rendered purely real (non-reactive) if the length of thetransmission line is set to be appropriately less than one-fourth of thewave length.

Assume that a transmission line whose characteristic impedance Z_(o) isequal to √50 Z_(L) is connected to the IDT transformer, and then Z_(in)is equal to 50 ohms. Thus, the necessary matching is attained.Specifically, the impedance of the load on IDT, whose characteristicsare indicated at "b" in the Smith Chart of FIG. 2, can be matched byadding a transmission line whose characteristic impedance Z_(o) is equalto √50 Z_(L).

A coaxial cable, parallel feeder or microstrip is not appropriate to thematching use-as described above. Instead, a transmission linetransformer, which can be advantageously reduced in size, is appropriatefor the purpose. Specifically, two transmission line sections are woundon a toroidal core, permitting reduction to the desired operatingfrequency band thanks to the reduction of wave length inspite ofreduction of size and shortening of the transmission line.

FIG. 5 (a) shows that a transmission line is connected to an IDT. FIG. 5(b) shows that a transmission line transformer as a transmission line isconnected to an IDT. Point markings in FIG. 5 (b) indicate the startingends of the windings. The windings of the forward and backward-goingline sections are wound in opposite directions, thereby causing themagnetic fluxes to cancel each other in the toroidal core.

Now, two different transformers are considered. One is a transmissionline transformer described above and shown in FIG. 6, and the other(FIG. 7) is a transmission line-like transformer whose windings arewound in the opposite way to those of the former one (FIG. 6).

The former (FIG. 6) shows the same characteristics as a transmissionline whereas the latter (FIG. 7) shows the same characteristics as atransmission line, and at the same time, it has an inductance four timesas large as the single winding due to the sum magnetic flux in thetoroidal core. The inductance L which is required for matching, is builtin the transmission line, as seen in FIG. 8.

A transmission line transformer has an impedance conversion function anda balanced-to-unbalanced conversion function whereas a transmissionline-like transformer has an inductance component "L" built therein anda balanced-to-unbalanced conversion function. The phases at places A andB in FIG. 8 were found to be 180 degrees apart from each other at theoperating frequency.

A surface-acoustic-wave device according to a preferred embodiment ofthe present invention will be described below.

FIG. 1 shows a surface-acoustic-wave convolver as comprising an Si(semiconductor) base plate 20, an SiO_(z) film 21, an ohmic electrode22, aluminum layers 23, a Z_(n) O film (piezoelectric substrate) 24, agate electrode 25, IDT electrodes 26 and transmission-like transformers28 using toroidal cores. Each transmission-like transformers 28 isconnected to an associated IDT by wires 27 as a matching andbalanced-to-unbalanced converting circuit. Each IDT electrode has anunderlying conductive layer 23 at ground potential.

The windings of the forward- and backward-line sections L, and L₂ in thetransmission-like transformer in FIG. 9 are wound on a toroidal T_(L) inopposite directions, thereby providing the sum magnetic flux in thetoroidal core.

The windings of the forward- and backward-line sections L₁ and L₂ in thetransmission-like transformer in FIG. 10 are wound on a toroidal T₂ inone and same direction, and these windings are connected to exteriorcircuit so as to permit current to flow in one and same direction in thewindings, thereby providing the sum magnetic flux in the toroidal coreT₂.

These different type transmission-like transformers when used as atransmission line and matching circuit, are essentially a two-terminalcircuit, and then the inductance as seen from the input towards theclosed or short-circuited output end is selected so as to cancel thesusceptance ωC_(r) of the IDT. The admittance of the IDT when matchingis completed is indicated at "c" in FIG. 2.

As may be understood from the above, the following advantages resultfrom the present invention:

1) A π-circuit or T-circuit which includes L, C and R therein, are usedfor matching a conventional SAW circuit to an associated exteriorcircuit, and accordingly the number of the required parts for matchingis two or more. In contrast, a single transmission-like transformerattains matching and balance-to-unbalance conversion in an SAW deviceaccording to the present invention. Usually, an SAW convolver has twoIDTs, and accordingly the number of the required parts for matching andbalance-to-unbalance conversion can be reduced from six to two, therebypermitting the reduction both of manufacturing cost and size and weightof an SAW device.

2) The problem of appearance spurious or noise signals will beincreasingly conspicuous with the reduction of an SAW device in size. AnSAW device according to the present invention uses a toroidal core, andsignals in the form of magnetic flux are confined in the toroidal core,and therefore undesired waves are prevented from radiating anddisturbing a nearby device. Also, the toroidal core prevents exteriorspurious or noise signals from invading and disturbing the signalmagnetic flux in the toroidal core. Thus, the transmission line-liketransformer constitutes a matching circuit which is free of spurious ornoise signals.

As for the leakage of magnetic flux from a toroidal core, such leakageflux can be reduced to minimum or negligible small by evenly windingconductor on the toroidal core.

3) The use of a transmission line-like transformer in an SAW devicepermits conversion of an unbalanced circuit including a grounding lineto a balanced circuit, thereby attaining balance-to-unbalance conversionalong with impedance matching. This permits elimination of expensivebalanced-to-unbalanced transformers from an SAW device, and accordinglythe reduction of manufacturing cost and size.

I claim:
 1. A surface-acoustic-wave device comprising:a semiconductorbase; a piezoelectric substrate laid on said base; at least onetransducer laid on said piezoelectric substrate for converting betweenelectrical signal energy and acoustic wave energy traveling across saidsubstrate and having at least two electrical terminals, said transducerpresenting a given electrical impedance at said terminals; and a singleimpedance matching unit having a single core and a pair of transmissionlines wound on said core and connected to said transducer, said linesbeing magnetically coupled to said core and wound thereon so thatmagnetic fields are produced in the same direction by means of currentsflowing into said lines so as to cancel the capacitive resistance ofsaid transducer and to provide a balance-to-unbalance convertingfunction.
 2. A device according to claim 1 wherein said transmissionline has a length of less than one-quarter line wave length at thefrequency of the induced acoustic wave.
 3. A device according to claim 1wherein said core is a toroidal core.
 4. A device according to claim 1wherein said lines are configured as a coaxial line.
 5. A deviceaccording to claim 1 wherein said device includes at least oneinterdigital electrode pair.